According to U.S. Pat. No. 3,652,488 and the corresponding DE-C 20 34 919, flame resistance is obtained by addition of platinum and the flame-resistant characteristic is enhanced by addition of carbon black. As a result, these compositions are naturally black. The pale shades so often desired for joint-sealing compositions cannot be produced.
U.S. Pat. No. 3,821,140 and the corresponding DE-C 23 00 504 and U.S. Pat. No. 3,677,999 describe flame-resistant or self-extinguishing compositions which are prepared by addition of metal oxides or hydrates of metal oxides, such as hydrated aluminum oxide. At elevated temperatures in the course of a fire, water is split off from these products, and although this briefly reduces flammability, under the elevated temperatures which occur during a more prolonged fire it contributes toward destabilization of the dimethylpolysiloxane by hydrolysis. On dissociation, basic oxides, such as magnesium oxide or aluminum oxide, are formed from these hydrates of metal oxides, and intensify the decomposition under these conditions. The silicone matrix then disintegrates completely due to depolymerization. Combinations of metal oxides/hydrated metal oxides with graphite are known from U.S. Pat. No. 4,405,425 and corresponding EP-B 40 750.
According to DE-B 29 09 462 and DE-A 30 41 031, flame-resistant polysiloxane compositions are prepared by addition of halogenated diphenyl compounds, for example octabromodiphenyl ether. In the event of a fire however, polyhalogenated dibenzofurans or dibenzodioxins are formed from these products, and these are undesirable because of the known toxicological problems. The use of these products might therefore be possible to only a limited extent in future.
None of the above mentioned methods lead to products which increase in volume in the event of a fire and thus show a so-called intumescence effect According to U.S. Pat. No. 5,262,454 and corresponding DE-A 40 13 161 and U.S. Pat. No. 4,694,030 and corresponding DE-A 36 02 888, this can be achieved by addition of expandable graphite compounds and other additives, such as nitrogen-containing polyphosphates, or by hollow glass beads. However, because of the presence of graphite, the products are black and no pale shades can be produced. Furthermore, the surfaces of the silicone compositions are rough and unattractive due to the addition of the expandable graphite or the hollow glass beads, because these products are not available in the necessary small particle sizes (less than 20 .mu.m).
Boron compounds are used in silicone compositions because of their neutron-absorbing action. The use of boron carbide for this purpose is known from U.S. Pat. No. 4,176,093.